Entity class defines a table, and each instance of this class represents a row in the database table. It has mappings to tell Room how it intends to present and interact with the information in the database.

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@Entity annotation marks a Data class as a database Entity class. For each Entity class a database table is created to hold the items. Each field of the Entity is represented as a column in the database (unless it is denoted otherwise, check Entity docs).

Every entity instance that is stored in the database must have a primary-key.

Example ​

import androidx.room.ColumnInfo
import androidx.room.Entity
import androidx.room.PrimaryKey

@Entity(tableName = "item")
data class Item(
	@PrimaryKey(autoGenerate = true)
	val id: Int = 0,

	@ColumnInfo(name = "name")
  val itemName: String,

	@ColumnInfo(name = "price")
  val itemPrice: Double,

	@ColumnInfo(name = "quantity")
	val quantityInStock: Int
)

fun Item.getFormattedPrice(): String {
    return NumberFormat.getCurrencyInstance().format(itemPrice)
}

The default value of id = 0 is necessary, since the id parameter serves as the primary-key.

The @PrimaryKey annotation is needed as well. autoGenerate makes Room to generate the ID for each entity.

The @ColumnInfo annotation is used to customise the column associated with the particular field (like specifying the custom column name).

Item.getFormattedPrice() is an extension function used to convert a double value to the desired currency format. Normally you don't want to change an entity class that represents data just to format the data (SRP).